Gastric bypass surgery, in which the stomach is stitched into a tiny pouch, has long been seen as a last resort for the dangerously obese. Doctors attributed rapid post-surgical weight loss to reduced hunger and restricted eating resulting from the smaller stomach.

But new evidence suggests weight loss may result when the procedure alters the types of microbes in the gut.

Scientists reached this conclusion by transferring microbes from bypass-treated obese mice to a group of lean mice raised in sterile conditions that left them with no intestinal bacteria at all. Two weeks after the transfer, recipient mice had lost considerable weight; another group that received microbes from obese mice in a placebo group — undergoing surgery without gastric bypass — stayed the same.

The new microbiota may trigger weight loss, says gastroenterologist Lee Kaplan of Massachusetts General Hospital, by sending chemical signals that cause the human host to burn more calories, which helps use up the body’s fat reserves.

“The physiology of the whole body changes in response to bypass surgery — it’s not just a matter of making the stomach smaller so people eat less,” says Kaplan. “If we can uncover which changes are responsible for what the surgery does, we can devise less invasive treatments,” including drugs or microbial transplants that would set the microbiome right.

Gut Bacteria May Help Shed Pounds

More evidence that the bacteria that live within us may be partly responsible for whether we’re fat or thin: In a novel experiment at Washington University, researchers implanted bacteria from pairs of human twins — one obese and the other lean in each pair — into the intestines of genetically identical mice bred in a sterile environment so that their guts contained no microbial life.

After several weeks, the mice given microbes from the fatter siblings grew heavier, gaining up to 17 percent more body fat than the ones with gut bacteria from the thin twins. (Those mice remained svelte.)

Since the mice were genetically identical, the study eliminated hereditary differences as a variable. The study also controlled environmental factors and therefore provided clear evidence that the weight variation was due to change in gut bacteria and nothing else.

“The next step is to identify the actual microbes that are responsible,” to learn how the bacteria affect weight change, says Washington University’s Jeffrey Gordon, the senior investigator for the study.

[This article originally appeared in print as "Gut Surgery Spawns Slimming Microbes."]